DESCRIPTION: Through a division of labor, the mammalian visual system is capable of analyzing the components of a complex visual stimulus. In the visual system of macaques, such a division incorporates two well- recognized and parallel groups of retinogeniculocortical neurons (designated M and P) and a third, recently discovered and neurochemically distinct group, analogous to the K channel of other primates. Whereas the functional organization of M and P channels is understood at a reasonably precise level, permitting evidence of normal and pathological states in humans to be interpreted in their context, the macaque K channel and its implications for visual function across primates remain largely a mystery. Studies are outlined in this proposal which will examine the role of the macaque K channel by determining the following: l) the precise origin of this channel in the retina by a combination of retrograde labeling of ganglion cells that project to each of six intercalated LGN layers with intracellular filling of the cells with small molecular weight, visible markers; 2) the functional properties of LGN neurons in the K channel, intercalated between cells of the principal layers, by quantitative electrophysiological methods during presentation of defined visual stimuli; 3) the quantitative contribution to intercalated LGN circuits from the retina and a second source of visual drive, the superior colliculus, by stereological analyses of electron microscopic data; 4) the level of convergence and divergence of intercalated neurons to their cortical targets, the cytochrome oxidase blobs in VI, by retrograde transport of different tracers from blobs functionally characterized by optical imaging; 5) the relative power of the geniculocortical component of the K channel assessed by comparing in strict quantitative fashion the synaptic terminations of geniculate axons in layers IVCbeta (P channel), IVCalpha (M channel) and the blobs (K channel). At the core of these studies are the hypotheses that the third channel is, itself, broken up into three distinct components, each characterized by a pair of anatomically distinct layers in the LGN and each carrying physiologically distinct inputs to VI, and that these geniculocortical are sufficiently powerful to provide a dominant visual drive to neurons of the blobs. From these studies will be determined the place of a third channel in visual function of a primate species whose visual system closely resembles that of humans. A reasonable expectation is that a more complete understanding of the K channel will permit a more precise delineation of each retinogeniculostriate channel's role in human vision, both normally and under pathological conditions.